[VHFcontesting] Tech Help needed

Sun Jan 3 12:17:57 EST 2021

I quit!

3 hours of messing with that antenna and I got nowhere. It's starting to snow. Maybe we'll get enough to bury them.

The theory behind moving the shorting bars out to more the freq up the band may be based on simple J pole type design, when you short higher up the driven element, it increases in resonate freq.
The movement of the shorting bars had no perceivable effect of center of band.

The 70 ohm (RG-6) phasing line might be able to raise the center freq if shortened. If anyone has done this, please reply with length to attempt.
Next step

The driven element might be able to be sleeved increasing the diameter and potential bandwidth, which is the biggest problem with this design.
No change

Repositioning D1 may help but this has unknown effect of performance.
I clamped D1 2 inches then 6 inches up the boom. Really sent SWR sky high

I have a 6L6 that I will use if I can't get anywhere. Very frustrated and cold.

73,
Paul Rollinson, KE1LI
Rollinson Associates
860-928-5147
860-208-8814 Cell/Text
Paulrollinson at sbcglobal.net

Representing Component Part 
Manufacturers for over 35 years

-----Original Message-----
From: David Olean [mailto:k1whs at metrocast.net] 
Sent: Saturday, January 2, 2021 11:42 AM
To: peter h; Paulrollinson at sbcglobal.net; vhfcontesting at contesting.com
Subject: Re: [VHFcontesting] Tech Help needed

I think the problem is that the Q of the antenna is so high that the elements cannot supply a reasonable reactance at the FT-8 frequency for the matching section to work with. The solution is to try to tweak the driven element or maybe director #1 to raise frequency a bit.  After all, if the elements are not resonant, then no amount of adjustment of the T match or any other type of matching system, will have any effect. 
I have not played with a 6M7, but do note that director #1 is quite important in coupling into a resonant set of elements.  As you get farther away from the driven element, the element dimensions are less important.One way to try a change is to drill and tap the element end so that you can add more metal if trimming the element shorter does not give you what you want. A small machine screw can add element length.

Dave K1WHS

On 1/2/2021 9:48 AM, peter h via VHFcontesting wrote:
> 0645 pst 02jan2021
> Perhaps contacting M2 directly, with a cc to K6MYC might provide the needed info.
> bt73Pete, N6ZE in DM04ne with an older 6M7 also
>
>
> -----Original Message-----
> From: Paul Rollinson <Paulrollinson at sbcglobal.net>
> To: 'VHF Contesting Reflector' <vhfcontesting at contesting.com>
> Sent: Sat, Jan 2, 2021 7:02 am
> Subject: [VHFcontesting] Tech Help needed
>
> Good morning all,
>
>   
>
> I hope you had a nice holiday.
>
>   
>
> Part of prepping for the January contest included taking down the 
> stack ofM2 6M7antennas and tuning them a bit higher to accommodate 
> FT8/4 frequencies where the swr runs close to 2:1
>
>   
>
> After several hours of tweaking we were unsuccessful in elevating the 
> frequencies. Moving the shorting bars had little effect and the 
> elements are crimped together so shortening them would be permanent; I 
> prefer not to do this.
>
>   
>
> After diving thru the specs, the antennas are 6M7 and not the modern 
> versions 6M7JHV. Below is a link to the manual.
>
> https://dl-mail.ymail.com/ws/download/mailboxes/@.id==VjN-j4MNikUvGG6d
> I72pGt 
> Fnbtnm78WZEHQ84kCP5dKOEa_RWizZIRV1VtWw7y3GBuOyPtJBEyjJRlraKdU25EkUFw/m
> essage 
> s/@.id==AOqprj4WDiqnX--apQiykLPEILk/content/parts/@.id==2/raw?appid=YM
> ailNor
> rin
> <https://dl-mail.ymail.com/ws/download/mailboxes/@.id==VjN-j4MNikUvGG6
> dI72pG 
> tFnbtnm78WZEHQ84kCP5dKOEa_RWizZIRV1VtWw7y3GBuOyPtJBEyjJRlraKdU25EkUFw/
> messag 
> es/@.id==AOqprj4WDiqnX--apQiykLPEILk/content/parts/@.id==2/raw?appid=Y
> MailNo 
> rrin&ymreqid=117033b5-9242-646b-1c92-e7000301a500&token=zitEzqOML3j84e
> 6ealFT 
> T5U7-km5qEQF52lp7AcCuBZYNpCJiKFonMipX0nRikauD7BKLkpZAtQ_aN6eUBTbi2UcMC
> 60Zo-i
> fhoLye8kBZhFaDq07_sCfvo7gxJ36C_A>
> &ymreqid=117033b5-9242-646b-1c92-e7000301a500&token=zitEzqOML3j84e6eal
> FTT5U7 
> -km5qEQF52lp7AcCuBZYNpCJiKFonMipX0nRikauD7BKLkpZAtQ_aN6eUBTbi2UcMC60Zo
> -ifhoL
> ye8kBZhFaDq07_sCfvo7gxJ36C_A
>
>   
>
> I'm looking for advice. The manual states that moving the shoring bars 
> out will increase the tuned frequency (which doesn't make sense to 
> me). If this is true, I may experiment with either adding push on 
> rings to act as capacitance hats, electrically lengthening the t 
> match, or swedge on lengths of rod to extend the t matches.
>
>   
>
> The design freq of the antenna is very sharp: 49.5 to 50.5MHZ but I 
> can't seem to stay within 2:1 at 50.315. Running 1 kw is an issue for the amp.
>
>   
>
> The other, not preferred option is to run a short segment of 70 ohm 
> coax as a transformer but I will lose output/receive in the form of heat.
>
>   
>
> We hung the antennas about 10 ft in the air to sweep so the numbers 
> are fairly accurate.
>
>   
>
> Any help or suggestions would be appreciated.
>
>   
>
> Thanks,
>
>   
>
> Paul Rollinson, KE1LI
>
> FN41AU
>
> 860-928-5147
>
> 860-208-8814 Cell/Text
>
>   <mailto:Paulrollinson at sbcglobal.net> Paulrollinson at sbcglobal.net
>
>   
>
>   
>
>                    Happy Holidays
>
>   
>
>
>

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